1. Field of the Invention
The present invention relates to a valve gear or valve drive system of an engine, and particularly, to a valve drive system that includes a switching mechanism that performs switching between a plurality of cams having different valve lift characteristics.
2. Description of the Related Art
Conventional techniques for a valve drive system of an engine are disclosed in, for example, Japanese Published Examined Patent Application No. H2-43004, Japanese Patent No. 3365805, and Japanese Translation of International Application No. 2006-520869.
A valve drive system disclosed in Japanese Published Examined Patent Application No. H2-43004 includes a low-speed rocker arm that is pressed by a low-speed cam that is used for a low speed, a high-speed rocker arm that is pressed by a high-speed cam that is used for a high speed, and a switching mechanism that performs switching between the cams to be used. In this valve drive system, an intake valve or an exhaust valve is connected only to the low-speed rocker arm.
The switching mechanism includes a hydraulic piston that moves between the low-speed rocker arm and the high-speed rocker arm. The hydraulic piston is stored in the low-speed rocker arm when the low-speed cam is used. The hydraulic piston is engaged both with the low-speed rocker arm and with the high-speed rocker arm when the high-speed cam is used.
A valve drive system disclosed in Japanese Patent No. 3365805 includes a switching mechanism that performs switching between two kinds of cams. The switching mechanism includes a roller guide supported by a rocker arm so as to be movable in the axial direction thereof, a roller rotatably supported by the roller guide, and a cam mechanism that moves the roller guide in the axial direction. The roller is in contact with either of the two kinds of cams. The cam mechanism includes a rail groove and an annular groove that are formed on a camshaft, a follower pin disposed at the roller guide so as to be able to enter or leave these grooves, and a return spring that returns the roller guide to an initial position. A terminal of the rail groove is connected to the annular groove.
In this valve drive system, the roller guide and the roller move toward one side in the axial direction by allowing the follower pin to move forwardly to be fitted into the rail groove, and one of the two kinds of cams is connected to the rocker arm. On the other hand, the roller guide returns to the initial position while receiving a spring force of the return spring by allowing the follower pin to move backward, and the other one of the two kinds of cams is connected to the rocker arm.
A valve drive system disclosed in Japanese Translation of International Application No. 2006-520869 includes a switching mechanism that moves two cams having different valve lift characteristics in the axial direction of a camshaft. The switching mechanism includes a cam carrier formed of a cylindrical body that has the cams, spiral grooves formed at both ends of the cam carrier, and a pair of driving pins that can be inserted into the spiral grooves, respectively. The cam carrier is supported by a main camshaft that penetrates the cam carrier. The cam carrier rotates together with the main camshaft, and moves toward one side in the axial direction of the main camshaft by allowing one driving pin to be inserted into one spiral groove. On the other hand, the cam carrier moves toward the other side in the axial direction by allowing the other driving pin to be inserted into the other spiral groove.
Each of the rocker arms of the valve drive systems disclosed in Japanese Published Examined Patent Application No. H2-43004 and Japanese Patent No. 3365805 includes the movable member (piston, roller guide) of the switching mechanism. Therefore, these valve drive systems increase in the mass of the rocker arm. Additionally, the rocker arm has a complex structure, and hence has a possibility that a portion of the rocker arm may have a low rigidity. If the rocker arm is great in mass and is low in rigidity, a cam action cannot be reliably transmitted to the intake valve or the exhaust valve during a high-speed operation. In this case, the opening/closing timing and the amount of valve lift may become inaccurate, thus resulting in damage to the valve drive system.
Additionally, the valve drive systems disclosed in Japanese Published Examined Patent Application No. H2-43004 and Japanese Patent No. 3365805 cannot control the moving speed of the movable members (piston, roller guide). Therefore, the movable members moving at a high speed collide with a stopper part, and an impact sound occurs.
The high-speed rocker arm of Japanese Published Examined Patent Application No. H2-43004 is always pressed against the high-speed cam by a lost motion spring. The follower pin of Japanese Patent No. 3365805 is pressed against a side wall of the annular groove by the return spring in a state of having moved to the inside of the annular groove. In other words, in the valve drive systems of Japanese Published Examined Patent Application No. H2-43004 and Japanese Patent No. 3365805, there are components that are pressed against a rotating part on the camshaft side and that are brought into slide contact therewith, and therefore a loss in engine power occurs.
The cam carrier and the main camshaft of the valve drive system of Japanese Translation of International Application No. 2006-520869 are splined to each other. Therefore, the connection structure formed by the cam carrier and the main camshaft is complex, and production costs are high.
In the valve drive system of Japanese Published Examined Patent Application No. H2-43004, Japanese Patent No. 3365805, or Japanese Translation of International Application No. 2006-520869, a switching mechanism is needed for each cam (i.e., each cylinder). Therefore, if the valve drive system is used for a multi-cylinder engine, the number of switching mechanisms becomes larger correspondingly to a rise in the number of cylinders, and production costs become higher.